1. Field of the Invention
The present invention relates to an electromechanical motor, especially a piezoelectric actuating drive.
2. Prior Art
The cockpit of a motor vehicle attempts to achieve an optimum interplay between design and technology. Various pointer instruments are located within the field of view of the driver. These pointer instruments must meet different technical requirements and have a competitive price for mass production of motor vehicles. The “Messwerk 2000” by the company Siemens VDO is an example of such a pointer instrument.
The “Messwerk 2000” is a four-pole stepper motor drive geared down by a single-stage worm gear drive. The stepper motor is driven by two sinusoidal coil current variations, phase shifted to one another by 90° in phase angle, as a function of time. A sign of the phase shift determines a direction of rotation and a frequency determines a speed of rotation of the motor shaft. Within one full period of 360° of the sinusoidal current variations, up to 128 intermediate stages can be set in a reproducible manner. The utilization of these intermediate stages is called microstepper operation.
A complete “Messwerk 2000” actuating drive containing the stepper motor characterized above consists of 12 single parts. The stepper motor is composed of two coils with a common stator plate and a permanent-magnet rotor. With regard to the component costs, the coils and permanent magnet account for most of the costs. Apart from the material costs, the production costs are also decisive for the prices which increase approximately proportionally as the number of components of the actuating drive increases.
In EP 1 098 429 B1, an electromechanical motor operates without rotating coils. During operation, a drive ring is displaced whilst rotating by an operation, offset in time, of at least two electromechanical drive elements so that, by a direct transfer of force from the drive ring to the shaft, which is located within the drive ring, the shaft is rotated. The rotating displacement movements of the drive ring can be produced by an actuator which, e.g., is driven piezoelectrically, magnetostrictively, electrostrictively or electrodynamically, so that an actuating drive which is better suited for mass production with regard to material costs and production costs is obtained. The piezoelectric actuators are attached to the drive ring in such a manner that their respective displacement acts radially on the drive ring in which context, if necessary, other measures are also taken to achieve an introduction of force on the drive ring which is as symmetric as possible.
Although the technical circumstances mentioned last result in a drive optimized with regard to function, having the best constant-velocity characteristics (constancy of rotational speed independently of the instantaneous position of the shaft) without torque fluctuations, the longitudinal extent and the radial arrangement of the bending actuators entail that the planar drive variants require considerable space in the plane and in most cases appear to be not very compact. However, compact planar drives are urgently needed due to the very narrow installation space conditions given in cockpit instruments.